Low-temperature dehydration of well fluids



HIS ATTORNEY Oct. 1, 1963 D. c. MEYERs LOW-TEMPERATURE DEHYDRATION 'OF WELL FLUIDS Filed April 18. 19Go INVENTORI DOUGLAS C. MEYERS BY: @M/m mm vn. d mv l mv N Nv ov v N Ir l Il I III .m 1-f,| m.|.|I lll. IIIIIII P I l Il Il l lVml. S Hw EN fNN t", a .NP m a n A. m. am ON a n w .HII w .l 'L Q h @N m mf v @V3 United States Patent C 3,105,855 LGW-TEMPERATURE DEHYDRATION F WELL FLUlDS Douglas C. Meyers, Metairie, La., assigner to Shell il Company, New York, NX., a corporation of Delaware Filed Apr. 18, 1960, Ser. No. 22,993 Claims. (Cl. 260-676) The invention relates to improvements in the method and means for Athe low-temperature dehydration of highpressure well iiuids which consist largely of gas.

'Ihe invention is, more particularly, concerned with the dehydration of well uids having a high gas-to-liquid ratio by cooling the gas to a temperature at which moisture is precipitated, together with condensible hydrocarbons, if present.

The low-temperature separation of moisture from such well lluid has become accepted practice and is necessary to reduce the moisture content and, in `some instances, the liqueable hydrocarbon content, to below levels to render the gas merchantable. To separate such constituents by precipita-tion involves a depression in the uid temperature, as by expansion through an orifice or isentropic expension in an engine. Unless the source iiuid has a very high pressure, e.g., above 1,200f2,000 lbs. per square inch, it is desirable to recompress the dehydrated gas, In some cases (as described; for example, in the U.S. patent to Maher, No. 2,873,814) expansion and recompression are eiected by a turbine-compressor set, so that the mechanical or shaft work generated in the expansion is used to drive the compressor and produce a product stream having a pressure only one to several hundred pound-s per square inch below that of commercial or sales pipe lines, e.g., 600 to 1200 pounds per square inch.

When moisture is precipitated by such a cooling operation difficulties are encountered due to Ithe formation of hydrates, which are crystalline compounds or agglomerates of water and certain components of the gas; they look like hard snow and are formed as a resul-t of temperature reduction, particularly when high-pressure gas containing water vapor is expanded to a lower pressure. It is known to melt such hydrates by heating the liquid stratum into which they are precipitated within a separator vessel; Ithis is likewise described in the said patent to Maher, and the use of the well uid for this purpose is suggested in the U.S. patent to Wilson, No. 2,818,454. However, it is often the case that the temperature of the well fluid, prior to the pressure reduction, is too low to impart suticient heat to the liquid stratum to melt the hydrates and it is then necessary to add heat, usually by means of a gas-fired heating vessel. Normal safety practices dictate that fired vessels such as heaters must be 1ocated 100-200 feet from other production facilities and at off-shore installations this usually requires the use of a separate platform which may be very costly.

It is lthe object of the invention to provide an improved method and separator for dehydrating well iluid by expansion and .temperature reduction wherein the need for a red heater is obviated despite the fact that the initial temperature of the gas may be so low that it cannot be practicably used by usual techniques to melt the hydrates.

A more specific object is to provide an improved lowtemperature dehydration method and separator employing the principle of the heat pump, whereby heat can be added to the system at relatively low temperature, e.g., ambient temperature, and wherein the gas is re-compressed after expansion to the low pressure necessary for attaining the low temperature.

A further object is to provide an improved method and separator for the low-temperature dehydration of well uid which is economical as regards power requirements and can be applied to well fluid which is initially at too low a pressure to permit expansion to attain the separating temperature by simple expansion through an orifice and at too low a temperature to supply the heat required to melt the hydrates.

ln summary, according Ato the invention at least the predominantly gaseous portion of the gas fluid is expanded to attain a low temperature at which hydrates are or may be formed by an expansion which includes substantially isentropic expansion, eg., in a gas expansion turbine with the production of shaft work; the expanded gas is separated into gaseous and non-gaseous constituents in a separator vessel; the cold gas is discharged from the vessel and the .temperature and pressure thereof are raised by compression, preferably by using the said shaft work for all or part of the compressor load and, preferably, by adding heat at ambient temperature to the cold gas prior to compression; and the resultant heated and re-compressed gas is passed in heat exchange to the nongaseous phase in the separator Vessel to melt the hydrates therein. It will be understood that re-compression of the gas is usually to a pressure which is somewhat below that of the source gas, but that the invention is not restricted to any specific relation of the final pressure to the pressure of the source gas inasmuch as the final pressure of the gas is dictated in many instances by the requirement that the gas be able to enter pipe lines at the normal operating pressure thereof.

The invention will be described in detail with reference to the accompanying drawing forming a part of this specication the single View of which shows one preferred embodiment, elevation, parts being shown in section.

Referring to the drawing in detail, 1 represents a producing well or series of wells flowing by a manifold or other suitable connections through a iiow line 2. The

Well stream, which consists predominantly of gas, mayV be produced and passed into the ilow line 2 at well head pressure'or it may be reduced in pressure, as by a valvel 3, so as to enter the flow line 2 at a pressure considerably lower than the well head pressure. For example,

the well streammay pass into the line 2 at a pressure of several thousand pounds per square inch or pressures as low as 1,000 to 2,000 pounds per square inch, depending upon the operating pressure of the gas transmission line into which the dehydrated gas must ultimately discharge.

When the gas contains considerable amounts of liquid constituents, such as hydrocarbons and/or water, it is preferably fed through an open valve 2a into a preliminary, three-phase separator 4 which may be of any known or suitable type and contain trays S, 6, and liquidlevel controllers 7, '8, -to separate the stream into three phases, which are, respectively, a gas phase 9, a hydrocarbon condensate phase 10 and an aqueous phase 11. These phases are discharged through separate conduits 1'2, 13 and 14, respectively,

The gas, containing water vapor, passes at atemperature T1 and pressure P1 through an expansion-gas engine l or motor, such as a turbine 15, wherein it is expanded substantially isentropically and from which it is dischargedat a' reduced temperature Tg via a conduit 1-6. Inthe preferred operation the conditions T2, P2 do not lead to Vhydrate formation; however, When hydrate formation in the engine isa problem a hydratesuppressant, such as brine or a glycol such as diethylene glycol or triethylene glycol, may be in jected via a branch line 17, controlled by a valve 18. The gas may be further expanded through an expansion valve 19 and discharged into the upper region of l a main, horizontally elongate separator vessel 20 at a temperature T3 and pressure P3, which are below T2 and l P2, respectively. The valve 19 is advantageously mounted L Patented Oct. 1,1963

and pressure P2 immediately adjacent the vessel to minimize dimculties due to hydrate formation but may in certain cases be omitted. When the valve is omitted or ully open T2, and P2 are substantially the same as T3l and P3, except lfor changes due to heat absorption and pressure drop in the conduit 16. At the low temperature and pressure prevailing within the Vessel water vapor is condensed and hyldrates may be termed and, in most instances, liqueliable hydrocarbons are also condensed. 'These nongaseous constituents, including the hydrates when formed, are precipitated into the lower region of the vessel, wherein they stratify to form a liquid aqueous phaseV 21 at the bottom and a liquid hydrocarbon phase 22 immediately above the former, while the gas remains at the upper 4region 23. The hydra-tes are collected in either or both of the liquid phases. It should be understood that the hydrate-suppressant, when used, is normally admitted in amounts insuiiicient to prevent hydrate formation within the main separator vessel.

When the preliminary separator 4` is not fused the well stream is passed directly to the engine 15 via a branch conduit 24 by opening a valve 25 therein and closing a valve 2a in the conduit to the separator.

The hydrocarbon condensate from the preliminary separator is -fed through the conduit 13 to the upper part of the main separator vessel by ow through an expansion valve 26, which is advantageously mounted closely adjacent lto the vessel. This condensate should preferably not be injected into the lower, liquid-containing region of the vessel but into the gas 23 because a large portion of the condensate vaporizes upon expansion through the valve 216 and would refrigerate the liquid; this is contrary to the intent herein, inasmuch as it is desired to heat the liquid, as is described hereinafter. The aqueous phase 'from the preliminary separator is transferred by the Conduit 14 to any suitable region of the main separatorvvessel, such as a bottom inlet 27. Inasmuch as this water is not subject to appreciable vaporization and is at about the temperature T1, which is considerably above T3, it aids in Warming the aqueous liquid phase 21.

The cold gas ilows through the length of the vessel to a gas outlet 27a, and the cross sectional area of the gas phase Z3 is sufficiently large to permit substantially all non-gaseous constituents to' drop out into the liquid phases. The cold, dehydrated y.gas flows from the outlet via a conduit 2,8 to a heater 29. The latter may be a heat exchanger having a coil 30 through which a lluid such as air or water is circulated by a pump 31 and from which it is discharged at 32.V The warmed gas emerges via a conduit .33 at a temperature T 4 and substantially at the pressure P3; According to a 4feature of the invention the temperature T3 is below ambient temperature and T4 is ambient temperature or somewhat below; it thereby is feasible to use as the heating fluid which flows through the exchanger =or heater 29 a stream such as water -or air available at ordinary temperature. By thus absorbing heat at ordinary temperature, such as 50 to 100 F., use is made of the heat pump principle.

The warmed gas flows via the conduit 33 to a compressor 34 which is driven from the turbine 15 by a shaft 35. This re-compresses the gas and raises its temperature to a temperature T and pressure P4 which are greater than T4 and P3, respectively. In most instances all of the work required to drive the compressor is derived from the turbine 15. However, in instances in which the source well stream is at a pressure which is so low as to require fur-ther compression, added shaft work may be supplied by a motor 36, which may, for example, be a steam turbine as indicated or an elechric'motor. The recompressed gas `flows via a conduit v37 to a heat exchange lduct 38 mounted within the main separator vessel in the lower region so as to be immersed in the liquid therein. The recompressed gas heats the liquid phases 21 and 22 and thereby maintains them above let means for the liquid. In the prefered embodimentl it includes separate outlets 40 and 41 for the aqueous and hydrocarbon condensate phases, respectively, con-V nected to discharge conduits 42 and 43 provided with valves 44 and 4S, respectively. When so constructed the vessel contains suitable 1or known separating means, suchas a skimmer 46 which defines an overow Weir and catch basin to which the outlet 41 is connected and which contains a liquid-level controller 47 for controlling the discharge valve 45 lto maintain the desired level in the catch basin. The vessel further contains an inter-phase level controller 48 for sensing the height of the interf face between the strata 2l and 22 and controllingv the discharge valve 44 to maintain the level. Water from the conduit 42 is discharged to a water ydisposal system while condensate from the conduit 43 is ilowed totanks Y for utilization.

For convenience, the temperature and pressure relationships are summarized by the following inequalities:

of 1500 p.s.i. (pounds per square inch (P1) at a wellhead temperature oi F. (T1). Water, eg., sea water,

is available at an ambient temperature of 70 F., and the sales line pressure is 800 p.s.i. gage. Y By using a conventional low-temperature dehydration and recovery unit, wherein the -gas temperature is low- I ered by simple expansion through a throttling device (at constant enthalpy), a reduction in pressure from 1500 and l800 p.s.i. would eiect cooling to only 40 F. Only small amounts of moisture cou-ld be separated at this temperature.

By the use Iof the heat pump according to ythe invention far deeper cooling is possible. Thus, if P3, the pressure in 'the vessel 20, is 300 p.s.i., and the valve 1,9 is

fully open, so that P2=|P3, the following conditions would prevail:

A. Assuming an ideal process, with efficiency in the .turbine and compressor and reversible processes, the temperature T3 would be brought to -50 F. by the isentropic expansion and the gas would'give up 1250*` B.t.u. per pound-moleY as shaft energy. Heat absorbed in the heat exchanger 29v from seawater at 70 F. to

bring T4 to 60 F. would fbe 1950 B.t.u. per pound-molef; In compressing the gas isentropically from 300 toY 800 f p.s.i. (P4) the temperature Would be raised to 180?.

(T5), and this would require 1050 B'.t.u. per pound mole. Y

Flow through the heat exchange duct 3'8` in the lowtemperature hydrate section of the separator can lresult in a cooling of the gas to 50 F. (T6), this would make 1550'B.t.u. per pound-mole available for melting hy-V drates. Y

VB. In an actual operation, wherein the same Vpressure relationships prevail (assuming 70%V turbine eiciency' with 10% heat absorption in the turbine and pipes, and 70% compressor eihciency with 10% heat radiation from compressor andpipes), the temperature T3 Vwould be h-37' vF.,'and 875 yB.t.u. per pound-mole of gas would be generated as shaft work in the expansion; 1050 B.t.u. per pound-mole 4are absorbed from ambient, low temperature vessel and exchanger 29 to bring T4 'to 60 F., and 'l the recompression requires 1500 B.t.u. per pound-mole and brings T5 to 180.

By way of example, consider a natural gas well pro-` ducing 'gas having a speciiic gravity of,0.7, at a pressure 1400 Btu. per pound-mole are available for melting hydrates with a final 'temperature y It may be noted Ithat the expansion valve 19 is provided primarily for controlling the speed of the turbine 15 (or equivalent engine) and al-so to prevent the formation of hydrates within the turbine when this becomes a problem, inasmuch as such hydrate formation could be harmful and result in reduced shaft work output from the turbine. For example, at 1500 p.s.i. the hydrate temperature can be approximately 72 F., or only =8 below the well stream temperature used in the example. Because the temperature drop due to the isentropic expansion is much greater than the drop in hydrate temperature due to reduced pressure, hydrates could in some cases begin forming at 1200 p.s.i. and 68 P., unless a hydrate suppressant such as a glycol is added. In the preferred operation, the valve 19 is throttled only to the extent necessary to attain its stated purposes. When the 'valve is throttled too far the temperatures T2 and T3 are raised, leading to reduced heat absorption in the exchanger 29, particularly when the latter receives heat from ambienttemperature air or sea water; this may be insuiiicient to melt the hydrates in the vessel 20 and could make it necessary to compress the gas in the compressor 34 to a pressure considerably above the sales line pressure, and expend the gas, after flow through the heat exchange duct 38, to the sales 'line pressure. In the foregoing example it was assumed that hydrate formation would not be a problem.

I claim as my invention:

l. The method of dehydrating a high-pressure well stream consisting predominantly of gas which comprises the steps of: expanding at least the predominantly gaseous portion of the said well stream with the production of mechanical work to cool the same to a temperature at which hydrates are formed, establishing within a coniined separating zone at least one horizontal liquid stratum, introducing said expanded, cold gas stream into said separating zone and therein precipitating non-gaseous constituents into said liquid stratum, withdrawing the residual gas from the separating zone, supplying heat to the withdrawn gas, re-compressing the withdrawn gas and thereby further heating the same, iiowing the heated, compressed :gas in indirect heat exchange with the said liquid stratum within the separating zone to warm the liquid stratum and maintain the same above hydrate-formation temperature for melting the hydrates therein, and discharging liquid of said stratum from the separating zone.

2. In combination with the steps defined in claim l, the steps of initially introducing the said well stream, prior to expansion, into a preliminary separating zone md therein separating the stream into gaseous, hydrocarbon condensate, and aqueous phases, supplying the gas from said preliminary separating zone to the said expansion step, and feeding the hydrocarbon condensate and aqueous phases separately to the first-mentioned separating zone by expanding the former and admitting it to the separating zone above the liquid stratum, and admitting the latter directly into the liquid stratum.

3. The method of dehydrating a high-pressure well stream consisting predominantly of gas which comprises the steps of expanding at leas-t the predominantly gaseous portion of said well stream substantially isentropically to cool the same to a temperature below ambient temperature and suiciently to precipitate water, introducing the expanded cold gas stream into a confined separating zone and therein stratifying the stream into a gas stratum and at least one liquid stratum, withdrawing gas from the gas stratum lof the separating zone at a temperature below ambient temperature, supplying heat to the withdrawn gas at substantially ambient temperature to warm the same, re-compressing the -warmed gas and thereby further heating the same, iiowing the further-heated, compressed gas in indirect heat exchange with the said liquid stratum within the separating zone to warm the liquid stratum and maintain the same above hydrate-formation temperature,

6 and discharging liquid of said stratum from the separating zone.

4. The method of dehydrating a high-pressure well stream consisting predominantly of gas which comprises the steps of introducing said stream into a preliminary separating zone and therein separating said stream substantially without pressure reduction into three phases, respectively a gas phase, a hydrocarbon condensate phase and an aqueous phase; establishing within a coniined, horizontally elongated separating zone two horizontal liquid strata, respectively a liquid hydrocarbon stratum and an aqueous stratum, at a pressure below that or said preliminary separating zone; discharging the condensate phase separately from the preliminary separating zone, expanding the same, charging the same into said elongated separating zone at a level above said strata, and precipitating liquid constituents into said strata; separately transferring the aqueous phase from the preliminary separating zone to the aqueous stratum within the elongate separating zone; withdrawing the gas from :the preliminary separating zone, expanding it with the production of mechanical work to a pressure above that of said elongated separating zone to cool the same; further expanding the expanded, cold gas to further cool it to a temperature below ambient temperature and below the temperature at which hydrates are formed and introducing it into said elongated separating zone at a level above the liquid strata, and precipitating non-aqueous constituents, including any hyd-rates formed, into the liquid strata; withdrawing residual gas from the elongated separating zone and supplying heat thereto at ambient temperature to warm the same, re-compressing the warmed gas solely by using said mechanical work which was generated =by the expansion of the gas and thereby further heating the same; owing the further-heated, compressed gas in indirect heat exchange with the liquid strata in said elongated separating zone to warm the liquid strata and maintain the same above hydrate-formation temperature for melting the hydrates therein; and separately discharging liquid of said strata from the elongated separating zone.

5. A well-Huid separator `for separating an aqueous portion from the gaseous portion of a high-pressure well uid which comprises: a separator vessel; means for maintaining a liquid layer in the lower region of the vessel and a gaseous layer in the upper region of the vessel; a liquid outlet for said vessel; duct means immersed in said liquid layer for passage of a -fluid in indirect heat exchange relation to the liquid, said duct having an inlet and an outlet; a gas-expansion engine for expanding said `gas stream, said engine having a low-pressure outlet connected to feed expanded, low-temperature gas to said vessel; a gas outlet for said vessel; heating means connected to receive the gas discharged from said gas outlet and a compressor connected to re-compress the gas discharged from said heating means and having an outlet connected to the inlet of said duct for owing compressed and heated gas therethrough to warm the said liquid.

6. A separator as defined in claim 5 wherein said heating means comprises a heat exchanger having a passageway for the flow of said gas discharged from the vessel and an yisolated passageway for the iiow of a iluid in indirect heat exchange with said gas, and means for iiowing a uid at substantially ambient temperature through the latter passageway.

7. In combination with the separator as deined in claim 8, a preliminary three-phase separator for separating well iluid into three phases, being respectively a gaseous phase, a hydrocarbon condensate phase and an aqueous phase, said separator having an inlet for receiving said well fluid, means [for maintaining said three phases at different levels therein, and separate outlets for said phases, irst duct means for iiowing liquid from said hydrocarbon phase to the first-mentioned separator vessel into an upper region thereof; second duct means for 7 flowing liquid from said aqueous phase separately 1to said separator vessel into a lower region thereof; and third duct means for flowing gas from said gaseous phase to the intake of said expansion engine.

8. A well-fluid separator for separating an aqueous portion from the gaseous portion vof a high-pressure vwell tuid which comprises: a preliminary separator vessel for separating well fluid into three phases, being respectively a gaseous phase, a hydrocarbon condensate phase, and an aqueous phase, said separator having an inlet for receiving said well uid, means for maintaining said three phases therein, and separate outlets for said phases; a main separator vessel having means ffor maintaining separate liquid aqueous and hydrocarbon condensate phases in the lower region thereof and a gaseous phase in the upper region thereof; separate liquid outlets for said liquid phases; heat exchange duct means immersed in said liquid layers of the main separator vessel for passage of a uid in indirect heat exchange with liquid of said layers; rst duct means connected to the preliminary separator Vessel to receive the hydrocarbon condensate phase therefrom and feeding the san-1e into an upper part of the main separator vessel; second duct means for feeding the aqueous phase from the preliminary separator vessel to the lower region of the main separator vessel; a gas-expansion turbine; a compressor having a drive shaft coupled mechanically to said turbine; third duct means for iiowing gas from the preliminary separator vessel toY the intake of said turbine; fourth duct means for yfeeding the expanded, cooled gas from the turbine to an upper region of the main separator vessel; a gas outlet for said main separator vessel; fifth duct `means for iiowing gas from said lastamentioned gas outlet to the suction intake of said compressor; means for supplying heat toy the lgas flowing through said lfth duct means; and sixth duct means for -feeding compressed, heated gas from the discharge of said compressor to said heat exchange duct means for flow therethrough to warm the liquid layers.

9. A separator as defined in 'claim 8 wherein said firstk duct means includes an expansion valve situated near the main separator vessel for expanding said conde-,usate` phase.

10. A separator as defined in claim 8 wherein said fourth duct means includes an expansion val-'ve situated near the main separator for further expanding the gas from Ithe turbine.

References Cited in the le of this patent 

1. THE METHOD OF DEHYDRATING A HIGH-PRESSURE WELL STREAM CONSISTING PREDOMINANTLY OF GAS WHICH COMPRISES THE STEPS OF: EXPANDING AT LEAST THE PREDOMINANTLY GASEOUS PORTION OF THE SAID WELL STREAM WITH THE PRODUCTION OF MECHANICAL WORK TO COOL THE SAME TO A TEMPERATURE AT WHICH HYDRATES ARE FORMED, ESTABLISHING WITHIN A CONFINED SEPARATING ZONE AT LEAST ONE HORIZONTAL LIQUID STRATUM, INTRODUCING SAID EXPANDED, COLD GAS STREAM INTO SAID SEPARATING ZONE AND THEREIN PRECIPITATING NON-GASEOUS CONSTITUENTS INTO SAID LIQUID STRATUM, WITHDRAWING THE RESIDUAL GAS FROM THE SEPARATING ZONE, SUPPLYING HEAT TO THE WITHDRAWN GAS, RE-COMPRESSING THE WITHDRAWN GAS AND THEREBY FURTHER HEATING THE SAME, FLOWING THE HEATED, COMPRESSED GAS IN INDIRECT HEAT EXCHANGE WITH THE SAID LIQUID STRATUM WITHIN THE SEPARATING ZONE TO WARM THE LIQUID STRATUM AND MAINTAIN THE SAME ABOVE HYDRATE-FORMATION TEMPERATURE FOR MELTING THE HYDRATES THEREIN, AND DISCHARGING LIQUID OF SAID STRATUM FROM THE SEPARATING ZONE. 